1. Field of the Invention
The present invention relates to image forming apparatuses such as electro-photographic apparatuses, printers and the like, and an image forming apparatus to get color developed images using a revolver type developing unit.
2. Description of the Related Art
In image forming apparatuses to get color images according to an electro-photographic system such as color copiers, printers and the like, for achieving the downsizing, an apparatus is developed, which forms latent images corresponding to yellow (Y), magenta (M), cyan (C) and black (BK) on a photosensitive drum by rotating the drum plural times and supplies toners in different colors to the drum from plural developing devices visualizes the formed color images and obtains a full-color image by superposing the visualized toner images on a photosensitive drum or an intermediate transferring substance.
Further, a revolver type developing unit is developed to supply toners in different colors to latent images formed on a photosensitive drum when the drum is rotated. This revolver type developing unit is equipped with 3 developing devices containing yellow (Y), magenta (M) and cyan (C) toners and further, a developing device for black (BK) toner added.
For this revolver type developing unit, there are two methods available for removing developing devices from a revolver during the maintenance including a periodic inspection; one method is to pull out developing devices in the longitudinal direction from the opening of the front frame of the main body of an image forming apparatus after rotating the developing unit to a prescribed position and the other method is to take out developing devices in the direction vertical to the shaft of a revolver from the side frame of the main body of an image forming apparatus after rotating a developing unit to a prescribed position.
In both methods, it is necessary to prevent a developing unit from rotating in the reverse direction for a malfunction when a developing unit is unbalanced when removing developing devices from a revolver. In Japanese Patent Application No. 11-316479, a device is disclosed to prevent the rotation of a developing unit in the reverse direction. This device prevents the rotation of a developing unit that is positioned using a stepping motor by engaging a plunger into a notch or prevents the reverse rotation of a developing unit by providing a one-way clutch to a central shaft of a developing unit. Thus, the reverse rotation of a developing unit when removing developing devices is prevented.
However, in the above-mentioned conventional image forming apparatus, developing devices are removed by stopping a developing unit in the state with voltage or current applied to a stepping motor for driving the developing unit. Therefore, for example, even when a developing unit is blocked to rotate by a plunger or a one-way clutch, there is still the possibility for generating abnormal rotation for a malfunction of a stepping motor.
Furthermore, the above-mentioned conventional image forming apparatus is in such a structure that a one-way clutch is installed to the central shaft of a developing unit and the one-way clutch is easily worn by the rotation of a developing unit, the clutch accuracy drops for its wear and tear and a certain play may be produced during the operation of the one-way clutch. Therefore, for example, when the central shaft of the developing unit rotates in conjunction with the rotation of a developer conveying auger of developing devices, a play is produced when the one-way clutch is operating, and when the developing unit rotates in the reverse direction even if only slightly, not only safety in the developing device removing work is impaired but also the developing devices can be damaged.
Accordingly, a highly reliable image forming apparatus to obtain developed color images using a revolver type developing unit is desired, which is capable of surely preventing the counter-rotation of the developing unit in the removing work of developing devices from a revolver, securing safety during the maintenance and preventing the damage of developing devices held by the developing unit.